I think this draws an non-parallel comparison. Even from a visual standpoint, the difference seems a little more obvious to me: one focuses on photos, the other on words/characters. Both, however, focus on the sharing of ideas.

A fundamental difference certainly exists in that certain types of ideas are more easily sharable in the form of images, and others in the form of language. This certainly creates a defining divide in the type of ideas shared on each platform. But if not to share ideas/sentiments, what would be the point of either platform?